The present invention relates to a light-shielding protective mask for protecting eyes and face skin of the worker from the arc light during welding.
Such an electrically controlled variable light transmission mask has been known as having a filter of variable light transmission coefficient and a controller adapted to change the light transmission coefficient upon detection of an arc light or arc current such that the light transmission coefficient is reduced when the arc light exixts and increased when there is no arc current. This kind of protective mask is advanategously permits observation of state of welding during suspension of the arc light without detaching the mask from the face to permit the worker to conduct the welding while checking the welding condition as required.
An example of this kind of mask hitherto proposed employs a liquid crystal cell driven by a driving circuit which is started upon detect of the arc light or the welding current by which the arc is generated. The reduction of light transmission coefficient, therefore, is commenced with a certain time lag (about 0.2 sec) to he generation of the arc. Therefore, within the period of this time lag, the strong light comes into eyes through the filter the light transmission coefficient of which has not been reduced yet.
This light, however, does not seriously damage eyes partly because the liquid crystal cell usually incorporates an ultraviolet ray absorption filter and partly because the time length of application of light is so large. However, this application of light makes the worker dizzy, so that the worker has to take a necessary precaution measure such as to close his eyes, avert his eyes away from the welding object and so forth, at least at the moment just before the start of welding. This seriously hinders the efficiency of the welding work in which the arc light goes on and off frequently.
The liquid crystal cell shows only a small range of change of light transmission coefficient. For instance, a liquid crystal having the minimum transmission coefficient of 0.002% has the maximum transmission coefficient of 3%. The worker, therefore, is obliged to inspect the minute portion of welded part under insufficient illumination.
It has been proposed also to reduce the light transmission coefficient just before the generation of arc by operating a foot switch on the floor. This, however, is not practical because it is quite troublesome to move the foot switch in accordance with the movement of the worker. This countermeasure, therefore, is not suitable when the welding has to be done over a wide range.